The behavior of trileaflet bioprostheses used for aortic valve replacement will be investigated. Commercially available bioprostheses will be marked with radiopaque markers at various points and will be implanted in the experimental animals. The movement of various parts of bioprostheses would be studied under x-ray. These measurements would be correlated with the measurements of blood pressure, blood flow and stiffness of the prosthetic valve leaflets. These observations would be used to determine the cause of pressure gradient and the amount of energy loss across these valves. The leaflet configuration would be studied and the stresses in the leaflets would be calculated. Based on the abnormal stresses an attempt would be made to predict the probable site and cause of fatigue failure of prosthesis. Previous investigation of the natural aortic valve has suggested that te aortic valve is a dynamic structure in which the leaflets and their supporting structure work together as a unit. The efficiency and longevity of the aortic valve appear to be due to low stiffness of the leaflets and absence of compressive stresses in the leaflets. Comparison of the behavior of bioprostheses with that of the natural valve should suggest possible modifications of bioprostheses that will improve their lives and performance and also define their limitations.